1. Field of the Invention
The present invention relates generally to electrotherapy devices and, more particularly, to an electrotherapy device that applies sequentially low energy current pulses for improved defibrillation efficacy.
2. Related Art
External defibrillators are used to provide electrical shocks to treat patients for a variety of heart arrhythmia such as ventricle fibrillation. Ventricle fibrillation is an uncoordinated contraction and relaxation of the individual fibers of the heart that produces no blood flow and results in death unless corrective measures are applied within minutes of onset. The most widely accepted clinical procedure to reverse the life-threatening condition of ventricular fibrillation is transthoracic electrical defibrillation in which an electrical pulse is applied to the surface of the thorax through electrodes, commonly called paddles or pads.
The use of electrical shocks to terminate ventricular fibrillation entails passing electrical current through the myocardium so as to restore the heart to its natural heart rhythm. For example, external defibrillators such as portable automatic or semi-automatic external defibrillators (generally, AEDs), typically provide a single, high-energy shock to a fibrillating patient through a pair of electrodes attached to the patient""s torso. If successful, the shock converts ventricular fibrillation to a normal sinus rhythm. However, the energy that must be delivered with the electrical shock to achieve defibrillation can produce deleterious effects, ranging from transient conduction abnormalities to myocardial necrosis. This is particularly true for pediatric patients whose body mass is a fraction of the typical adult patient.
Generally, manual external defibrillators are configured by a trained operator for the particular patient and patient condition, including the energy level to be delivered by the electrical shock. In contrast, automatic external defibrillators make such determinations for the device operator by using a fixed energy or escalating defibrillation energy protocol. Many of today""s AEDs also adjust varius aspects of the defibrillation waveform (such as time duration) based on patient impedance. Such an impedance measurement may provide useful information for certain purposes such as to estimate the impedance of the entire defibrillator path, heart rate, respiratory rate and other physiological parameters. However, such an approach fails to provide the information necessary to make an accurate estimation of patient size and mass. As a result AEDs often generate a single shock that may be optimal for larger adult patients but potentially damaging to smaller patients. Most AEDs today are not qualified for use on pediatric patients for this reason.
What is needed, therefore, is a system and method that can effectively defibrillate patients of varying body mass automatically and without a trained operator""s intervention.
The present invention provides a transthoracic electrical defibrillation method and apparatus for effectively defibrillating a fibrillating heart by delivering successive low energy electrical shocks to the fibrillating heart with the second such shock being applied at the onset of reinitiate fibrillation. Such low energy shocks produce current densities of sufficient magnitude to simultaneously place only a portion of myocardial cells in a refractory state, that portion being less than that necessary to defibrillate the heart. The composite effect of two or more low energy shocks each applied synchronously with the initiation or reinitiation of fibrillation, results in the successful defibrillation of a fibrillating heart.
Specifically, application of a first low energy shock induces a current density sufficient to depolarize a portion of the myocardial cells, this portion being typically less than that necessary to enable the myocardium to return to a normal sinus rhythm. In response to the first shock, then, some cells depolarize and enter a refractory state while other cells do not depolarize and remain non-refractory.
The polarization charge of those cells that remain non-refractory despite the first current pulse varies from cell to cell. This residual charge that remains in a cell membrane can subsequently cause the cell to depolarize. This spontaneous depolarization of such partially stimulated cells is referred to as reinitiate fibrillation. That is, fibrillation activity will reinitialize subsequent to the application of the first low energy pulse due to the depolarization of cells that received lower defibrillation current densities. These cells will depolarize on their own within a relatively short time after the defibrillation pulse and become refractory.
At the onset of such ECG activity, a second low energy electrical shock is administered. This second low energy shock will maintain the initially depolarized cells in their refractory state. Some of the initially depolarized cells will have yet to begin to repolarize prior to application of the second shock. Those cells will be maintained in their refractory state in response to the second shock. Any initially-depolarized cells that do begin to repolarize after the first shock will depolarize once again in response to the second shock. In addition, the second shock will possibly depolarize additional cells that have since attained a convertible current density. This depolarization will occur synchronously with the depolarization of the lower current density cells that depolarized spontaneously, marking the beginning of reinitiate defibrillation. The cells that are in the refractory state in response to the sequential pulses, in combination with the naturally depolarizing cells, form a critical mass of refractory myocardial cells, enabling the heart to return to a normal sinus rhythm.
Thus, by applying a subsequent low energy pulse to coincide with the first signs of is fibrillation, the sequential pulses of the present invention place the myocardium in a state in which it can respond to a normal sinus rhythm without the myocardial cells attaining current densities of the magnitude produced by conventional defibrillation pulse. That is, by taking advantage of the heart""s propensity to reinitialize fibrillation, the present invention supplies sufficient energy to achieve defibrillation while minimizing the peak current applied to the myocardium. This reduces the likelihood of the numerous adverse effects of high current densities. For example, the likelihood of myocardial necrosis decreases, increasing the probability of patient survival. In addition, it may be feasible that patients of all sizes may be defibrillated using one or more low energy current pulses that are safe and effective for all patients from pediatrics to adults. This would enable an AED to implement a single shock protocol that optimally defibrillates all patients. Furthermore, such lower energy currents result in lower applied voltages that can reduce size, weight and cost of the implementing external defibrillator.
A number of aspects of the invention are summarized below, along with different embodiments that may be implemented for each of the summarized aspects. It should be understood that the summarized embodiments are not necessarily inclusive or exclusive of each other and may be combined in any manner in connection with the same or different aspects that is non-conflicting and otherwise possible. These disclosed aspects of the invention, which are directed primarily to systems, methods, data and techniques related to the effective defibrillation, are exemplary aspects only and are also to be considered non-limiting.
In one aspect of the invention a method for defibrillating a fibrillating heart is disclosed. The method includes applying a first low energy current pulse to the heart; detecting a reinitiate fibrillation of the heart subsequent to the application of the first low energy current pulse; and applying, at the onset of reinitiate fibrillation, a second low energy current pulse. Preferably, the method also includes detecting a reinitiate fibrillation of the heart subsequent to the application of the second low energy current pulse; and applying, at the onset of reinitiate fibrillation, a third low energy current pulse.
In another aspect of the invention, an external defibrillator to defibrillate a patient""s fibrillating heart is disclosed. The external defibrillator includes an energy delivery system constructed and arranged to deliver sequentially two or more low energy electric shocks to a patient through at least two electrodes applied to the patient; a patient monitoring circuit constructed and arranged to monitor the patient""s heart rhythm; and a controller that determines whether the heart is fibrillating and that causes the energy delivery system to apply a first low energy electric shock upon detection of an initial fibrillation, and to apply a second low energy electric shock upon detection of a reinitiate fibrillation.
In a further aspect of the invention a transthoracic electrical defibrillation system is disclosed. The transthoracic electrical defibrillation system effectively defibrillates a fibrillating heart by delivering two successive low energy electrical shocks to the fibrillating heart with the second such low energy shock being applied at the onset of reinitiate fibrillation.
Various embodiments of the present invention provide certain advantages and overcome certain drawbacks of conventional defibrillation techniques. Not all embodiments of the invention share the same advantages and those that do may not share them under all circumstances. This being the, the present invention provides numerous advantages including the noted advantage that patients of all sizes may be defibrillated using one or more low energy current pulses that are safe and effective for all patients from pediatrics to adults. These and other features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.